Food service system utilizing reflected infrared signals to identify type of dish

ABSTRACT

In a food service system, prepared meals including chilled foods are assembled in individual meal trays (10) and held in the trays for a period prior to regeneration to prepare the food for heating. A control system of a rethermalizing trolley (12), which accommodates a large number of trays, can discern for itself which dishes (30) in the trays need heating and which are to remain cool. Dishes for hot and cold foods have white and black markings respectively, which results in reflected infra-red signals from sensors (42, 44) of the control system to be of discernably different strengths, so identifying the hot food dishes. Dish temperature is continually monitored by the control system by means of thermal contact with thermistors (46), which enables the system to oversee food temperatures both prior to and during regeneration.

.Iadd.This application is a continuation of prior copendinginternational PCT Application No. PCT/GB89/01400, filed Nov. 23, 1989,the benefit of the filing date which is hereby claimed under 35 U.S.C.§120. .Iaddend.

Food service systems are known, particularly for use in hospitals andother institutions where large numbers of meals have to be prepared andserved, which enable complete meal trays to be prepared some time beforethe meals are wanted, and food in dishes on the trays to be heated upsubsequently (in situ on the trays) for serving at the appropriate time.To preserve it during the intervening period, between preparation andserving, the food may be kept suitably cool by refrigeration. Suchre-heating of food from a chilled condition to prepare it for serving,is commonly referred to as regeneration or rethermalization. Suchsystems may also find use in transport catering (e.g. inflight onaircraft) and "meals on wheels" distribution services.

Apparatus for use in one such regeneration system is described forexample, in Patent Specification U.S. Pat. No. 4,285,391. That apparatuscomprises a rethermalizing trolley which can be stored in arefrigeration cabinet when loaded with meal trays, and wheeled out fromthe cabinet when the meals are to be served. The trolley comprises astack of shelves, each adapted to support one meal tray. Each shelfcomprises three flat heating elements and food dishes on the meal trayproject through openings in the bottom of the tray so as to stand on theelements. At an appropriate time the heating elements are switched on toheat the three dishes of each tray, to regenerate the food in thedishes, and the trolley is afterwards wheeled out from the cabinet forservice of the meal trays. The meal trays have insulated covers and theindividual dishes can be heated up, within the cabinet, withoutsubstantial detriment to the refrigerated environment which maintainsother (unheated) dishes cool.

Each meal tray for use in that apparatus (and as in some similar form isgenerally the case in all such regeneration systems) is formed toprovide a plurality of recesses to accommodate, for example, cutlery,condiments, and small dishes, and also three open-bottom compartments inwhich the dishes for hot food are retained. The three open-bottomcompartments are in registry with the heating elements of the trolleyshelf in one orientation of the tray on the shelf, but in case thereshould be no food regeneration needed for a particular tray it ispossible for each tray to be shelved in an opposite orientation whichresults in the heating elements for that shelf not being activated.However, should a tray be suitably inserted for activation of theheating elements then necessarily all three elements of that shelf willoperate, even though there may be no dish in one or more of theopen-bottom compartments.

A system which affords greater versatility, in allowing for meal trayswhich require some but not all of the heating elements for a tray to beactivated, is described in Patent Specification U.S. Pat. No. 4,005,745.In that system the heating elements of each shelf of the service trolleycan be switched on or off individually by an operator, so ensuring thatonly those heating elements which are actually needed are operated.

Similarly, in the system described in Patent Specification U.S. Pat. No.4,203,486 the heating elements can be controlled individually, in thatcase from a programmed computer control facility.

All three of those known systems so allow, to a greater or lesserdegree, for variations in the requirements for heating dishes in the hotfood compartments of the trays of a loaded trolley. The second and thirdsystems referred to permit appropriate individual control of all theheating elements, but in all three cases an operator has either toinspect each tray, or be supplied with the relevant information aboutthe make-up of the trays, in order to know the detail of the heatingrequirements. In the third system, for example, a printed record of therequirements is prepared in order that the rethermalizing control systemcomputer can be programmed.

There is a system described in patent specification U.S. Pat. No.4,584,466 in which the heating elements can only be activated in thepresence of a dish which actually requires heating. This is achieved byproviding the heating elements in the bases of the dishes themselves (sorequiring specially constructed dishes) and using conductors anddish-engaging contacts in the tray to supply power from support rails tosuch installed dishes; any dish without a heating element thereforeremains unheated, regardless of its position in the tray. Quite apartfrom such a system requiring a special kind of dish, the supply of largeelectric currents through readily separable contacts, first from therails to the tray and then from the tray to the dishes, is likely togive trouble from sparking and furring of the contacts, and is generallyan undesirable arrangement.

It is an object of the present invention to provide improvedrethermalizing apparatus of the general kind illustrated by the systemsreferred to.

The invention provides, in one of its aspects, rethermalizing apparatussuitable for use in regenerating chilled food in a food service systemin which prepared meals including chilled foods are assembled inindividual meal trays and held in the trays for a period prior toregeneration to prepare the food for eating, the apparatus comprising aplurality of individually controllable heating units for heating food asnecessary in individual food dishes within a tray, and a control systemarranged to operate the heating units selectively so as to heat foodrequiring regeneration but not to heat food which it is required shouldremain cool, .[.characterised .]. .Iadd.characterized .Iaddend.in thatfor effecting operation of those heating units only which are associatedwith dishes holding food to be heated the control system is arranged toreflect radiated sensing signals off the dishes and to respond to thereflected signals, the use of food dishes which are suitably distinctiveas between hot food dishes for holding food which is to be heated andcold food dishes for holding food which is to remain unheated enablingthe control system to distinguish between the hot and cold food dishesfrom differences in the reflected signals.

The sensing signals may be infra-red radiation and, in a simple form,dishes for containing food to be heated may be white or marked suitablywith a white patch, and dishes which are to remain unheated black ormarked with a black patch, so that the reflected signals are ofdiscernably different strengths as between different categories of dish.A sensor unit comprising an emitter and a detector can conveniently belocated adjacent to a dish positioned for heating in order to reflect asignal off the dish and to pass an indication of the strength of thereflected signal to the control system which so discerns whether or notthe food in that dish is to be heated. The invention provides, inanother of its aspects, rethermalizing apparatus suitable for use inregenerating chilled food in a food service system in which preparedmeals including chilled foods are assembled in individual meal trays andheld in the trays for a period prior to regeneration to prepare the foodfor eating, the apparatus comprising a plurality of individuallycontrollable heating units for heating food as necessary in individualfood dishes within a tray, and a control system arranged to operate theheating units selectively so as to heat food requiring regeneration butnot to heat food which it is required should remain cool,.[.characterised.]. .Iadd.characterized .Iaddend.in that the controlsystem comprises means whereby the temperature of food in the dishes ina tray can be monitored prior to and during regeneration.

The temperature of the food may conveniently be monitored indirectly bymonitoring the temperature of dishes holding the food in the trays. In aconvenient arrangement, a sensor unit comprising a thermistor or othertemperature-responsive electrical element is located adjacent to a dishpositioned for heating in order to be responsive to changes in thetemperature of the dish and to pass a corresponding signal to thecontrol system.

Monitoring the food temperature can be of advantage both in ensuringthat the food is kept under appropriate temperature conditions prior toregeneration, and in controlling the regeneration process for suitabletreatment of the food and energy conservation.

There now follows a detailed description, to be read with reference tothe accompanying drawings, of a food service system which illustratesthe invention by way of example.

In the accompanying drawings:

FIG. 1 is an overall view in perspective of a rethermalizing trolleywith one of a plurality of meal trays shown installed on a shelf of thetrolley;

FIG. 2 is a diagrammatic plan of compartments of a meal tray;

FIG. 3 is a view in .[.cross-section.]. .Iadd.cross section.Iaddend.through part of a meal tray, showing a hot food dish inposition in the tray;

FIG. 4 shows one sensor unit of a kind utilized in connection with eachof a plurality of heating elements of the trolley;

FIG. 5 is a schematic circuit diagram of control circuitry of thetrolley;

FIG. 6 is a sensor unit circuit diagram;

FIG. 7 is a schematic illustration of a sensor matrix switchingarrangement; and

FIG. 8 is a circuit diagram showing in more detail a matrix used tocontrol individual operation of heating elements from a common a.c.power source.

In a food service system, for example for distributing large numbers ofpre-prepared meals to the patients in a hospital, use is made of coveredmeal trays 10 in which complete meals can be arranged for service andheld for a time prior to eating, and rethermalizing trolleys 12 in whichthe trays can be stacked and by means of which chilled food in the trayscan be regenerated at the required time.

Meals may be prepared in a single central kitchen where also theindividual meal trays 10 are made up in accordance with menu selectionsmade by the patients. As illustrated by FIGS. 2 and 3, a lower deck 14of each meal tray provides an assortment of recesses 16, 18 and 20 foraccommodating, respectively, a drinking vessel, cutlery, and a side dishor plate (for, perhaps, bread and/or butter, cheese etc.). Also providedare three heating compartments 22, 24 and 26 in which dishes can beplaced for food that may need regenerating after chilling; asillustrated, the compartment 22 would be to hold a main meal dish orplate, and the compartments 24 and 26 could be for soup and hot pudding,respectively. However, as will become apparent hereinafter, dishes canbe placed in any of the heating compartments 22, 24 and 26 with foodthat is not to be hated up before eating, without any disadvantage; forexample, there may be a hot main course dish in compartment 22 and adish for a pudding to be eaten cold in compartment 26.

As can be seen from FIG. 3, each of the heating compartments 22, 24 and26 (only compartment 24 is shown) has a bottom wall formed by analuminium heat transfer plate 28 which is retained in an opening throughthe bottom of the lower deck 14 of the tray. The transfer plate 28 is inthe form of a flat disc on which a food dish 30 stands when lodged inthe compartment 24. The bottom surface of the transfer plate 28 isslightly above the surrounding bottom surface of the tray, a slightrecess so being provided in the tray bottom beneath the plate.

Each meal tray comprises also an upper deck 32 which provides a coverfor the loaded lower deck 14, and each of the two decks of the tray isfilled with a thermally insulating polyurethane foam filling 34 in orderto help maintain chilled food in the tray at a suitably low temperaturefor safe storage prior to being regenerated for eating.

Alternatively, at least the lower deck 14 of the tray may be filled witha so-called "eutectic" liquid, the tray being frozen before loading andenabling food to be kept for up to twenty-four hours in the tray priorto regeneration.

The prepared meal trays, with foods that require it being chilled intheir dishes in the trays, are stacked on shelves 36 of the servicetrolley 12. In the trolley illustrated in FIG. 1, there are eightshelves accessible from each of opposite sides of the trolley and eachshelf can accommodate two trays side-by-side; the trolley thereforecarries, when full, thirty-two meal trays.

In the upper surface of each shelf 36 there are six flat, disc-like,.[.aluminium.]. .Iadd.aluminum .Iaddend.heating plates 38 whichcorrespond in size and position with the heat transfer plates 28 of twotrays 10 installed on the shelf. The heating plates 38 are raisedslightly above the upper surface of the shelf in order to locate in thesimilar recesses in the bottoms of the trays, beneath the transferplates 28; the parallel heating plates 38 and heat transfer plates 28are so arranged to abut face-to-face. Electric heating elements 39(FIGS. 5 and 8) are arranged in a conventional manner for heating theheating plates 38, and so the heat transfer plates 28 and the fooddishes 30 standing on them in the trays 10.

Operation of selected heating elements at required times is controlledautomatically by an electronic control system of the trolley whichincludes a microprocessor. In order that the control system can knowwhich heating elements are required to be operated and can continuallymonitor the temperatures of the food dishes in the trays, the systemincludes sensor units 40 fitted into the heating plates 38.

As indicated by FIG. 1 (which shows only the heating plates for oneshelf, but all the shelves are in fact the same) each heating plate 38has an opening at its centre into which its associated sensor unit 40 isfitted. A flat top surface of the sensor unit is flush with the uppersurface of the heating plate.

Each sensor unit 40 (FIG. 4) comprises an infra-red emitter 42, aninfra-red detector 44 and a thermistor 46, the three components beingmounted on a printed circuit board 48 and potted together in a clearcasting resin. An uppermost edge of the thermistor 46 is coincident withthe top surface of the sensor unit so as to engage the undersurface ofthe associated heat transfer plate 28 of an installed tray; thethermistor is so installed to be electrically responsive to variationsin the temperature of the transfer plate 28. The three components 42, 44and 46 of the unit are electrically interconnected by the printedcircuit board 48 and three leads 50, 52 and 54 from the board providefor common ground, input and output connections respectively.

With reference to FIG. 5, the control system comprises in addition tothe microprocessor 100 (CPU) and system clock 101, a real-me (R.T.)clock 102 which enables the system to keep track of the time of day. Adigital display 103 and keypad 104, communicating with themicroprocessor through an interfacing chip 105 forming a peripheralinterface adapter (PIA), enable communication between the microprocessorand an operator. Memory capacity is provided through EPROM 106 and RAM107 facilities. An analog to digital converter 108 (ADC) enables voltagesignals from the sensor units 40 to be read by the CPU. A secondinterfacing chip 109 forming a versatile interface adaptor (VIA) linksthe CPU to two wiring matrices 56 and 58 for control of the heatingelements and the sensor units 40, respectively.

The matrices 56 and 58 enable the ninety-six heating elements and theninety-six sensor units (there being six of each on each of sixteenshelves) to be controlled individually. Each matrix is an 8×12arrangement of twenty distinct conductive paths considered in columnsand rows, each heating element or sensor unit being connected across aunique combination of column and row paths for individual control.

Considering the sensor units 40 in the matrix 58 first, the twelve rowpaths lead from a transistor switching device 60 and the eight columnpaths lead to an analogue switching device 62, both of which devices areunder the control of the VIA chip. Output signals from the analogswitching device 62 are passed to the ADC for conversion and onwardtransmission to the CPU.

Considering the heating elements in the matrix 56, the twelve row pathslead from a bank 64 of twelve triacs and the eight column paths lead toa bank 66 of eight triacs, both banks of triacs being under the controlof the VIA chip. Mains voltage is applied to .[.energise.]..Iadd.energize .Iaddend.the heating elements, but to prevent mainsvoltage reaching the CPU, the triacs are not directly connected but areisolated optically from the main circuitry by optotriacs. To increaseheating element life and reduce interference, the elements are switchedon and off only when the mains voltage cycle is at zero. The totalloading of all the heating elements of the trolley would be 288 kw, butby using a pulsed power-sharing technique the trolley is enabled tooperate from an ordinary 13 amp 3 kw power supply. The sensor units 40serve two purposes. As hereinbefore referred to, through use of thethermistors 46 they are adapted to signal variations in temperature ofthe heat transfer plates 28 and so (indirectly) to monitor foodtemperature. However, they further have an essential role in identifyingwhich dishes require heating and which do not when the time comes forregeneration. This latter function is effected through use of theinfrared emitter 42 and detector 44 of each unit.

Each heat transfer plate 28 has in it a small aperture 68 (FIG. 3) whichis so aligned with the sensor unit 40 of the associated heating plate38, when the tray is installed on a trolley shelf 36, that it forms awindow above the infra-red elements 42 and 44 of the unit. The sensorunit 40 can so "view" the bottom of any food dish 30 in the heatingcompartment 24 of the tray by reflecting an infra-red signal off thedish by means of its infra-red elements 42 and 44. [The thermistor 46 ispositioned aside from the window 68, so as still to engage the undersideof the heat transfer plate 28.] Each of the food dishes 30 used in thecompartments 24 of the tray is identified as one to be heated or not, asthe case may be, by means of a white or a black (respectively)identifying patch on its bottom surface opposite the window 68. Thereflected signal which the infra-red detector 44 of an activated sensorunit receives from a dish with a white marking is of a markedlydifferent level from that which would be received from a dish with ablack marking (or similarly in the absence of a dish from thecompartment) and the output signal from the sensor unit 40 as a whole isconsequently of a quite different order in the two cases. Accordingly,provided that in preparation of the meal trays dishes with whitemarkings are used to contain those foods which need to be regenerated,and dishes with black markings to contain those foods which do not, thecontrol system is able to determine automatically which heating elementsneed to be operated by distinguishing between the two levels of signalsbeing produced by the sensor units 40.

Power for the control system comes principally from an internalrechargeable battery, mains power being required only for the heatingelements during food regeneration, during which time the battery canalso be recharged.

The system is under the control of a program stored in the EPROM, whichprogram can be changed to vary or update the system when required, forexample to meet statutory regulations for the safe storage, handling andregeneration of chilled meals. When the system is first powered up, theprogram sets all the hardware to a predetermined set of conditions. Oncethis is done the program stays in a loop constantly reading the keypadto check for any input by the operator, and checking whether the timehas been reached for regeneration; if an input is detected the programwill branch off from the main loop to perform the required task.

Every two minutes the program jumps out of the main loop in order toactivate and read the sensors 40 (one by one) and so monitor thearrangement of food dishes 30 and their temperatures. This informationis recorded by the CPU in the memory. The CPU is thereafter able todetect any change in the arrangement and temperature of the food dishesby comparing the new information each time with that previouslyrecorded. One minute before it is time to regenerate, the programbranches off to make a last check of the food dishes prior toregeneration, and if all is satisfactory the program then branches offinto a regeneration routine.

Continual monitoring of the food dish temperatures prior to regenerationcan be important, as it may be essential that the temperature of chilledfood held in the trays does not rise above, say, 10° C. Should thecontrol system detect that such a rise in temperature has occurred itwill cause an alarm signal to be given together with an indication tothe operator as to which dish is affected.

During the regeneration routine, the system continually activates eachof the required heating elements in turn, also continually monitoringthe temperatures of the dishes. Towards the end of the routine, theheating elements associated with dishes which have already reached theirrequired temperature are activated by the control system only as much asmay be required to maintain that temperature.

This food service system so takes account of varying loads of mealcomponents to enable the system to use a minimum of energy duringregeneration. Regardless of the type of food being regenerated the finaltemperature of all the heated food compartments will be substantiallythe same.

Instead of individual programming of trolleys by keypads on each,collective programming by an infrared or other transmitter could ofcourse be employed.

Further discussion of the operation of the sensor units now follows.FIG. 6 is a circuit diagram for a single sensor unit, which may beconsidered as two variable resistors forming a potential divider, thetwo variable resistors being formed by the infra-red detector 44 and thethermistor 46. As a whole unit, the sensor can be considered as a devicethat produces an output voltage that is related to its temperature andposition relative to a reflective surface, i.e. the bottom surface ofthe dish 30.

The diode D1 is included to prevent the interaction of other sensors inthe matrix, by causing the signals to flow in one direction only.

As hereinbefore described and shown also in FIG. 7, the sensor units 40are connected so as to form a matrix. The matrix consists of twelvepower supply lines (the row paths) each one being controlled by thetransistor switching device 60, and eight signal lines (the columnpaths) connected to the analog switching device 62. The analog switchroutes the signal on one of the eight signal lines to the analog todigital converter (ADC). Exactly which signal line is selected dependson a binary number from 0-7 being sent to the analog switch by the CPU.

In order for the value of a particular sensor unit to be read, the CPUmust first switch on the transistor that powers the sensor unit inquestion through the corresponding power supply line, and then send therequired binary value to the analog switch to connect the output signalfrom the sensor unit, on the corresponding signal line, to the analog todigital converter; although there are eleven other sensor outputsconnected to this signal line, they are all switched off leaving onlythe signal from the sensor unit in question to be present. These twooperations actually happen simultaneously with the CPU sending a singlebinary value to the VIA that controls the sensor matrix.

It can be seen from this that only one sensor may be read at a time. Toincrease the accuracy, each sensor is actually read sixty-four times andan average taken.

The output from each sensor unit 40, once read by the CPU, is anumerical value between 0 and 255. This value is a representation of thetype and temperature of the dish 30 with which the sensor is associated.

Initially, before the trolley is loaded, all sensor units will return alow value (e.g. 20) there being slight variations between the ninety-sixreadings. Any value very different from the rest would indicate that aproblem exists, an object obscuring the sensor for example, and awarning buzzer would be sounded to alert the operator.

Once the trolley is loaded and the sensors have been read, all thevalues should have dropped due to the lower temperature of the dishes,whether requiring heating or not, or remained the same if no plate ispresent; sensor units associated with dishes that require heating,having white patches viewed by the sensor units, will return a valuerather lower than those that are to remain cold (i.e. those having blackpatches). Any rise in value (i.e. when the trolley is first loaded)would indicate a plate of too high a temperature, the operator beingwarned should this be the ease.

Subsequent readings, between the times of loading and regeneration, willshow a gradual increase in values as the temperature of the food on thedishes rises towards the ambient temperature, this being slowed by theinsulated tray. A wildly different value from the rest would pointtowards a problem with the tray, such as an improperly located lidreducing the amount of insulation and causing the greater increase intemperature.

From the readings before the trolley was loaded the CPU is able tocalculate a value that corresponds to the 10° C. limit that the foodmust remain below before regeneration. Each sensor reading is comparedwith this value and warning given should any exceed it.

During regeneration the readings will rise fairly rapidly with the foodtemperature. Readings are now compared with another predetermined valuethat indicates the 70° C. that the food must reach to be properlyregenerated.

The CPU is able to differentiate between food products of greatlydifferent heat capacities by their temperature readings duringregeneration, and regulates the amount of heating given to each dishaccordingly in order to complete the regeneration of all the meals atthe same time. However, in practice most food products require the sameamount of heating and little such intervention by the CPU generallyoccurs.

A more detailed description now follows, with reference to FIG. 8, ofthe control matrix 56 for the heating elements 39.

The twelve row paths a to 1 lead to the bank 64 of twelve triacs and theeight column paths A to H lead from the bank 66 of eight triacs. Mainss.c. voltage is applied to the matrix through the triac banks. Theparticular heating element 39 indicated in the drawing will, forexample, be activated only by the matrix combination Ce.

Whilst by means of such a matrix, a primary conductive path through asingle element is created by any selected combination of single columnand row paths, a plurality of secondary conductive paths are potentiallycreated through combinations of elements in series. This would result inpower wastage, and some heating of elements which are intended to remaininactive, unless measures are taken to block all such secondary paths.

By means of a pair of oppositely orientated diodes 70 and 72 at the a.c.input of the triac bank 66 at the head of the column paths A to H, themains current is split into its negative and positive components. Onecomponent supplies the four paths A to D and the other componentsupplies the paths E to H. Each of the transfer paths, between a row andcolumn, includes a diode 74 which is orientated appropriately to matchthe polarity of the column path to which it is connected. All twelve ofthe transfer paths connected to each of the columns A to H are thereforeunidirectional in the same sense. Since the creation of a secondaryconductive path requires one of two transfer paths connected to the samecolumn path, and one of two transfer paths connected to the same rowpath, to be a reverse current path, blocking of all such secondary pathsis achieved.

We claim:
 1. For use in a food service system in which prepared mealsincluding chilled foods are assembled in individual meal trays and heldin the trays for a period prior to regeneration in the trays to preparethe food for eating, rethermalizing apparatus comprising:(i) a meal trayforming a plurality of compartments in which removable food dishes canbe located; (ii) dish-heating means comprising a plurality of heatingunits which for effecting thermalization are positioned beneath fooddishes in said compartments in said tray and are independently operablefor selective heating or non-heating of said dishes as required; (iii)food dishes of a first type (hot food dishes) which can be located insaid compartments in said tray to hold food which is to be heated; (iv)food dishes of a second type (cold food dishes) which can be located insaid compartments in said tray to hold food which is to remain unheated;(v) heating control means operative to activate said heating unitsselectively so as to heat any hot food dish located thereover in saidtray but not to heat any cold food dish located thereover; said controlmeans comprising a plurality of sensor units which are associated onewith each of said heating units to reflect radiated sensing signals offdishes in said compartments in said tray and to respond to the reflectedsignals, said hot food dishes and said cold food dishes beingdistinctive as to the signals caused to be reflected from them wherebyenabling the control means to distinguish between them for operation ofthose heating units alone positioned beneath hot food dishes. 2.Apparatus according to claim 1 characterized in that the sensing signalsare of infra-red radiation.
 3. Apparatus according to claim 1characterized in that it comprises a rethermalizing trolley having aplurality of shelves on which trays can be lodged, each shelf comprisinga plurality of heating units which are arranged to be in registry withthe dishes in a tray installed on the shelf in order to heat thosedishes if required.
 4. Apparatus according to claim 1 in which saidheating units comprise thermally conductive heating plates and electricheating elements for heating the plates.
 5. Apparatus according to claim4 in which there are openings in said heating plates for said sensorunits to reflect sensing signals off dishes thereabove.
 6. Apparatusaccording to claim 5 in which said sensor units are mounted in saidopenings within said heating plates.
 7. Apparatus according to claim 5in which each sensor unit comprises an infra-red emitter and an infrareddetector which are potted together in a casting resin and mounted withinsaid opening.
 8. Apparatus according to claim 7 in which each sensorunit comprises also a thermistor which is potted together with saidemitter and said detector, an uppermost edge of said thermistor beingcoincident with a top surface of the unit and said top surface beingflush with an upper surface of said heating plate.